Conventional circuit switched telecommunication networks set up a connection between two subscribers along a path which may include one or more switches and a variety of network transmission elements such as a trunked wireline telephone system, an optical fiber link, a microwave link, a mobile phone (wireless) link, a cordless telephone link, etc. Circuit switching means that a route between the calling party and the called party is set up at the start of a call and is maintained for the call unless there is a requirement for path replacement. Depending on how the call is set up it may not be routed suitably for a variety of reasons, e.g. for cost to the calling subscriber, for transmission quality or for circuit efficiency. Path replacement relates to setting up a new path on an existing circuit connection in a telecommunications network when it is determined that the connection is not optimal. A conventional path replacement procedure (ANF PR) is described in the standards ISO/IEC 13863, First Edition Jan. 11, 1995, "Information Technology--Telecommunications and Information exchange between systems--Private Integrated Services network--Specification, functional model and information flows--Path Replacement additional network feature" and ISO/IEC 13874, First Edition Jan. 11, 1995, "Information Technology--Telecommunications and information exchange between systems--Private Integrated Services Network--Inter-Exchange signaling protocol--Path replacement additional network feature" in conjunction with ISO/IEC standard 13873, Mar. 17, 1995, "Information Technology--Telecommunications and information exchange between systems--Private Integrated Services Network--Inter-Exchange Signaling Protocol--Call Diversion Supplementary Services".
This ANF PR is an additional network feature (ANF) and is applicable to certain basic services defined in ISO/IEC 11574. It allows replacement of an active call connection (established call) by a new connection through a private integrated services telecommunication network (PISN), in particular networks configured as integrated services digital networks (ISDN). The replacement is used in order to obtain a more suitable path between the two parties involved in the call. Reasons for such a path replacement may be indirect cost reduction, change of bearer capability, poor quality of service or for maintenance actions, as explained in Appendix A of ISO/IEC 13863. For instance, ANF PR is supposed to be invoked on receipt of knowledge of a non-suitable connection. If that knowledge is not available ANF PR may be invoked automatically on every type of supplementary services call (SS call) or simple call which may not be optimal, e.g. after call modification due to call diversion or forwarding, call transfer, termination of an add-on conference, or a transfer-by-join created call. Supplementary services are defined in the standard CCITT Rec. 1.210, for instance. Further, ANF PR may be invoked on permanent calls (nailed-up calls) for instance when maintenance is required on some of the equipment in the path.
It is not specified how the above standards are to be implemented, in particular it is not specified how to obtain the indication of an unsuitable path nor how to gain knowledge necessary to decide whether or not path replacement is to be invoked. Further, some supplementary services may be implemented using different methods, some of which result in optimum paths and some which do not. As there is no way to distinguish between these equivalent methods, it is conventional to recommend automatic path replacement even in situations when this could be avoided. The result is unnecessary and useless replacement attempts on calls which have an optimal routing already. The ISO/IEC 13863 standard contains a standard ANF PR routine when the old connection is at least partly retained and even contains a useless routine of attempting ANF PR when the complete old connection is retained, i.e. the old connection cannot be improved (was optimum anyway) and the ANF PR is terminated successfully by retaining the old connection.
It is anticipated that as telecommunication systems become more intelligent, the variety of methods of carrying out supplementary services will increase, hence there will be more situations in which more than one method may be used for carrying out supplementary services. Further, manufacturers of telecommunications hard- and software supplying to an international market will need to deliver systems in which the various methods may be selected or deactivated dependent upon the target application.
It is an object of the invention to provide a telecommunication system, a method of operating such a system and a node suitable for such a system in which unnecessary call replacement procedures are reduced and thus to improve the efficiency of the system.
It is a further object of the invention to provide a telecommunication system, a method of operating such a system and a node for such a system in which, despite alternative methods of implementing a particular supplementary service, path replacement procedures are carried out efficiently.